Transient potassium currents play an important role in shaping the electrical properties of hippocampal neurons; increases in excitability associated with their inhibition have been linked experimentally to enhancement of synaptic transmission and to epilepsy. Embryonic mouse hippocampal neurons express two transient potassium currents, an A-current and a D-current showing slower activation and inactivation, that can be separated in conventional whole-cell gigaohm-seal voltage clamp recordings based on voltage dependence and pharmacological sensitivities. In dissociated cell cultures relative levels of A- and D-current expression are dependent on the degree of contact between neurons and underlying glial cells, with greater contact favoring A-current at the expense of D-current. Freely diffusible factors do not appear to be involved. We propose here to further investigate the mechanism(s) by which glia influence transient potassium current expression, and the potential significance of transient potassium current variation in modulating hippocampal neuron excitability. Specifically, we propose to: a.Determine the type(s) of glia competent to induce the pattern of transient potassium current expression characteristic of neurons growing on mixed populations of glia. The patterns of potassium current expression induced by monolayers enriched in astrocytes, oligodendrocytes, microglia or fibroblasts (purity assayed with antibodies against characteristic markers) will be assayed electrophysiologically. b.Identify the modulatory signal and/or produce antisera with blocking activity. First, the ability of isolated glial membranes as compared to living cells to support the on-glia pattern of current expression will be determined. Second, panels of modulators and/or cell surface and extracellular matrix components will be screened for activity on potassium current expression. Third, if a modulator is not identified, blocking antisera will be produced using techniques for suppression of response to background antigens if necessary. c.Evaluate the potential functional significance of variation in transient potassium current expression using a computational model of hippocampal neuron excitability. An existing model will be modified to incorporate two transient potassium currents, and used to examine the consequences of variation in A- and D-current expression levels on the action potential wave form, repetitive firing, and consequent calcium entry and accumulation.